[Qt-interest] tracking uniqueness

[Andreas Pakulat]

On 24.04.10 12:36:31, Oliver Heins wrote:
> Thiago Macieira <thiago at kde.org> writes:
> 
> > Em Sábado 24. Abril 2010, às 10.10.29, Oliver Heins escreveu:
> >> Andreas Pakulat <apaku at gmx.de> writes:
> >> > You seem to have a bit of misconception about this. There is no Editor
> >> > instance in your code. All you do is declare a class and a nested class,
> >> > both can be used without an instance of the other as far as your example
> >> > goes. In particular, this:
> >> > 
> >> > class Editor
> >> > {
> >> > 
> >> > public:
> >> >   class Token
> >> >   {
> >> >   };
> >> > 
> >> > };
> >> > 
> >> > Editor::Token t;
> >> > 
> >> > is perfectly valid compiling code. And there's no instance of Editor
> >> > generated here, there's only a single instance of Token.
> >> 
> >> So it's not possible in C++ to describe the idea: »Every instance of
> >> Token is held by an instance of Editor«?
> >
> > Of course it is.
> >
> > The counter is per editor, right? So it's a property of the editor.
> 
> Yes, but I have to hand over the id exlicitely.  I hoped to do something
> like this:
> 
> class Editor
> {
>         class Token
>         {
>         public:
>                 Token () : uniqId(++id_) { }
>         private:
>                 quint32 uniqId;
>         }
> 
>         quint32 id_;
> };
> 
> or to make id_ somehow static only to the instance of Editor.  
> 
> But both doesn't work.  Instead, I have to pass id_ explicitely in every
> constructor call:

How should they, there is no Editor instance in the above code. It looks
a bit like you're coming from a java background where an inline-declared
class can access member variables from the surrounding class. This is
not possible in C++. Any object A can only know about its own members and
any global variables. Access to another objects B members always has to
go through either a member variable of A for B or a global variable for
B(or a function returning B).

Andreas

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